The numerical definition of "high voltage" depends on context. Two factors considered in classifying a voltage as "high voltage" are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer to the voltage between two conductors of a system, or between any conductor and ground.
In electric power transmission engineering, high voltage is usually considered any voltage over approximately 35,000 volts. This is a classification based on the design of apparatus and insulation.
The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current—and distinguish it from low voltage (50 to 1000 VAC or 120–1500 VDC) and extralow voltage (<50 VAC or <120 VDC) circuits. This is in the context of building wiring and the safety of electrical apparatus.
In the United States 2011 National Electrical Code (NEC) is the standard regulating most electrical installations. There are no definitions relating to high voltage. The NEC covers voltages 600 volts and less and that over 600 volts. The National Electrical Manufacturer's Association (NEMA) defines high voltage as over 100 to 230 kV. British Standard BS 7671:2008 defines high voltage as any voltage difference between conductors that is higher than 1000 VAC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 VAC or 900 V ripple-free DC.
Electricians may only be licensed for particular voltage classes, in some jurisdictions. For example, an electrical license for a specialized sub-trade such as installation of HVAC systems, fire alarm systems, closed circuit television systems may be authorized to install systems energized up to only 30 volts between conductors, and may not be permitted to work on mains-voltage circuits. The general public may consider household mains circuits (100 to 250 VAC), which carry the highest voltages they normally encounter, to be high voltage.